Time-dependent properties of 3D-printed UHPC with silica sand, copper slag, and fibers

This study investigates the development and characterization of 3D-printable Ultra-High-Performance Concrete (3DP-UHPC) by optimizing its rheological and mechanical properties. The research examines the influence of fine aggregate gradation, incorporating silica sand (SS-GI: 2–1 mm, SS-GII: 1–0.5 mm, SS-GIII: 0.5–0.09 mm) and copper slag (CS-GI: 2–1 mm) as an alternative fine aggregate. Additionally, steel fibers are incorporated at dosages of 0.5 %, 1 %, and 2 % to assess their impact on extrudability and buildability. Following systematic printing trials, ten optimal mix compositions demonstrating superior buildability, shape stability, and extrusion efficiency are selected for in-depth analysis. The selected mixes undergo flowability assessments, extrudability evaluations, stress growth tests, compressive strength testing, and microstructural characterization to evaluate early-age properties. The experimental findings reveal that both static and dynamic yield stress increase over resting time, following the order CS-GI > SS-GI & GII > SS-GIII. Copper slag-based formulations exhibit enhanced rheological performance, which is attributed to improved particle packing density and reduced porosity. Moreover, both plain and fiber-reinforced copper slag mixes demonstrate superior compressive strength across the X, Y, and Z axes due to their vitreous microstructure and densified matrix. The anisotropic strength behavior is examined for copper slag replacements at 25 %, 50 %, 75 %, and 100 % substitution levels. Microstructural analyses indicate that early nucleation of calcium silicate hydrate (CSH) gel contributes to ultra-high-strength development. While steel fiber incorporation reduces flowability, it enhances rheological stability and printability through mechanical interlocking within the binder matrix. The research establishes copper slag as a viable alternative for sustainable applications for 3D concrete printing of UHPC.